André-Louis Danjon was a French

France

The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...

 astronomer

Astronomer

An astronomer is a scientist who studies celestial bodies such as planets, stars and galaxies.Historically, astronomy was more concerned with the classification and description of phenomena in the sky, while astrophysics attempted to explain these phenomena and the differences between them using...

 born in Caen

Caen

Caen is a commune in northwestern France. It is the prefecture of the Calvados department and the capital of the Basse-Normandie region. It is located inland from the English Channel....

.

Danjon devised a method to measure "Earthshine" on the Moon

Moon

The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

 using a telescope

Telescope

A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...

 in which a prism

Prism (optics)

In optics, a prism is a transparent optical element with flat, polished surfaces that refract light. The exact angles between the surfaces depend on the application. The traditional geometrical shape is that of a triangular prism with a triangular base and rectangular sides, and in colloquial use...

 split the Moon's image into two identical side-by-side images. By adjusting a diaphragm

Diaphragm (optics)

In optics, a diaphragm is a thin opaque structure with an opening at its center. The role of the diaphragm is to stop the passage of light, except for the light passing through the aperture...

 to dim one of the images until the sunlit portion had the same apparent brightness as the earthlit portion on the unadjusted image, he could quantify the diaphragm adjustment, and thus had a real measurement for the brightness of Earthshine. He recorded the measurements using his method (now known as the Danjon Scale

Danjon scale

The Danjon Scale of lunar eclipse brightness is a five-point scale useful for measuring the appearance and luminosity of the Moon during a lunar eclipse. It was proposed by André-Louis Danjon when he was measuring the Earthshine on the Moon...

, on which zero equates to a barely visible Moon) from 1925 until the 1950s.

Among his notable contributions to astronomy was the design of the impersonal (prismatic) astrolabe now known as the Danjon astrolabe, which led to an improvement in the accuracy of fundamental optical astrometry. An account of this instrument, and of the results of some early years of its operation, are given in Danjon's 1958 George Darwin Lecture to the Royal Astronomical Society (in Monthly Notices of the RAS (1958), vol.118, pages 411-431).

He was Director of the Observatory of Strasbourg

Observatory of Strasbourg

The Observatory of Strasbourg is an astronomical observatory in Strasbourg, France.Following the Franco-Prussian War of 1870–71, the city of Strasbourg became part of the German Empire. The University of Strasbourg was refounded in 1872 and a new observatory began construction in 1875...

 from 1930 to 1945 and of the Paris Observatory from 1945 to 1963 (see biographical notice at the website of the Observatoire de Paris
http://www.obspm.fr/histoire/paris/danjon.fr.shtml).

He was awarded the Prix Jules Janssen

Prix Jules Janssen

The Prix Jules Janssen is the highest award of the French Astronomical Society. Created in 1897 and awarded annually, it is usually given in alternate years to a French astronomer, and to an astronomer of another nationality. It is distinct from the Janssen Medal , which is awarded by the French...

 of the French Astronomical Society in 1950,
and the Gold Medal of the Royal Astronomical Society

Gold Medal of the Royal Astronomical Society

-History:In the early years, more than one medal was often awarded in a year, but by 1833 only one medal was being awarded per year. This caused a problem when Neptune was discovered in 1846, because many felt an award should jointly be made to John Couch Adams and Urbain Le Verrier...

 in 1958.

Danjon died in 1967 in Paris

Paris

Paris is the capital and largest city in France, situated on the river Seine, in northern France, at the heart of the Île-de-France region...

.